1. Field of the Invention
The present invention relates to head rests of a vehicle seat. More particularly, the present invention relates to head rests of a vehicle seat that can increase restraint performance for heads of passengers when a vehicle collision is sensed or predicted.
2. Description of the Related Art
A head rest of a vehicle seat that can increase restraint performance for a head of a passenger when a vehicle collision is sensed or predicted is already known. Such a head rest (an active head rest) is taught, for example, by Japanese Laid-Open Patent Publication Number 2002-142910.
The known head rest includes two portions, i.e., a forward movable portion and a rearward immovable portion, that are linked to each other via parallel linking members. In the known head rest, when the vehicle collision is sensed by a sensor, the parallel linking members are actuated based on a signal from the sensor so that the movable portion moves forwardly and upwardly from a rest position to a projected position. As a result, the movable portion of the head rest moves closer to the passenger head so that the passenger head can be held by the head rest. Thus, the passenger can be prevented from suffering a whiplash injury.
Typically, during the vehicle collision (in particular, during a back-side collision), the vehicle seat is rotated rearwardly around a rotational axis of a seat reclining mechanism. At this time, the passenger is moved rearwardly while falling backward due to the rotation of the vehicle seat. As a result, the head rest is typically positioned lower than the passenger head.
However, as described above, in the known head rest, the movable portion and the immovable portion are linked via the parallel linking members. Therefore, depending upon the strength of the impact, often times, the movable portion cannot timely or immediately move to a desired height in order to align with the lowered passenger head. Consequently, the passenger head cannot be suitably held by the head rest. This means that the passenger cannot be sufficiently prevented from suffering a whiplash injury.